This invention relates to a video movement detector and in particular it relates to a detector of the general type originally developed by the Commonwealth of Australia and described in the specification of Austrialian Letters Patent No. 432,885, inventor Kevin W. Boyle, which comprise directing a scanning device into the area to be protected and storing the information received repeatedly and detecting change in subsequent stored information, a scanning device in that case including a television camera directed at the area to be scanned and feeding the information so obtained to a storage type tube, the area being scanned at repeated intervals to record any significant change in the information stored, a comparison rate divider being used to increase successive signal difference by virtue of a greater time difference.
This device used the photoconductive target of a vidicon camera tube as a frame difference generator, and in principle the vidicon face plate was exposed uniformly to white light and the video signal from the surveillance camera applied to the beam current electrode for one frame in every ten.
If the frame difference signal exceeded a threshold level, the position at which this difference occurred in the display was stored and the difference signal was integrated at that point over a small elemental area.
An alarm circuit was triggered if the integrated signal exceeded a second threshold, detection being restricted to an area within the display defined by a rectangular window, the height, width and position of which could be set by the operator.
Progress in semi-conductor technology led to the development of a semi-conductor version of the movement detector and in that system the TV display was divided into a matrix of elemental detection zones having a selected width and being a selected number of rows high to give a large number of elemental detection zones.
A number of identical integrators were used with one assigned to each of the columns and the video signal from the surveillance camera was demultiplexed column by column into the integrators and integrated over a selected number of TV scan lines.
Outputs from the integrators were multiplexed in turn into a high speed analogue-to-digital converter during spaced scan lines and the digital output from the converter for each of the elemental detection zones was compared with its respective value which had been stored in a random access memory at a previous time.
If the difference betwen comparisons exceeded a value set by a sensitivity switch, a small strobe pulse was mixed into the video display to indicate which elemental zone was in error and an alarm was generated. The memory was periodically updated to compensate for slow changes of ambient light level and for thermal drifts within the TV camera. The detection of movement zone was defined by a rectangle surrounding blocks of elemental detection zones, the shape and position of which rectangle could be set by the operator.
It was convenient to use a matrix 15 columns wide and 44 rows high, giving a total of 660 elemental detection zones and each elemental zone could conveniently be 3 microseconds wide and 5 TV scan lines high.